This invention relates generally to surveying of boreholes, and more particularly concerns methods and apparatus which enable significant reductions in well survey time.
In the past, the task of position mapping a well or borehole for azimuth in addition to tilt has been excessively complicated, very expensive, and often inaccurate because of the difficulty in accomodating the size and special requirements of the available instrumentation. For example, magnetic compass devices typically require that the drill tubing be fitted with a few tubular sections of non-magnetic material, either initially or when drill bits are changed. The magnetic compass device is inserted within this non-magnetic section and the entire drill stem run into the hole as measurements are made. These non-magnetic sections are much more expensive than standard steel drill stem, and their availability at the drill site must be pre-planned. The devices are very inaccurate where drilling goes through magnetic materials, and are unusable where casing has been installed.
Directional or free gyroscopes are deployed much as the magnetic compass devices and function by attempting to remember a pre-set direction in space as they are run in the hole. Their ability to initially align is limited and difficult, and their capability to remember degrades with time and environmental exposure. Also, their accuracy is reduced as instrument size is reduced, as for example becomes necessary for small well bores. Further, the range of tilt and azimuthal variations over which they can be used is restricted by gimbal freedom which must be limited to prevent gimbal lock and consequent gyro tumbling.
A major advance toward overcoming these problems is described in my U.S. Pat. No. 3,753,296. That invention provides a method and means for overcoming the above complications, problems, and limitations by employing that kind and principal of a gyroscope known as a rate-of-turn gyroscope, or commonly `a rate gyro`, to remotely determine a plane containing the earth's spin axis (azimuth) while inserted in a bore-hole or well. The rate gyroscope has a rotor defining a spin axis; and means to support the gyroscope for travel in a bore-hole and to rotate about an axis extending in the direction of the hole, the gyroscope characterized as producing an output which varies as a function of azimuth orientation of the gyroscope relative to the earth's spin axis. Such means typically includes a carrier containing the gyroscope and motor, the carrier being sized for travel in the well, as for example within the drill tubing. Also, circuitry is operatively connected with the motor and carrier to produce an output signal indicating azimuthal orientation of the rotating gyroscope relative to the carrier, whereby that signal and the gyroscope output may be processed to determine azimuth orientation of the carrier and any other instrument thereon relative to the earth's spin axis, such instrument for example comprising a well logging device such as a radiometer, inclinometer, etc.
U.S. Pat. No. 4,192,077 improves upon 3,753,296 in that it provides for use of a "rate gyro" in combination with a free gyroscope, with the rate gyro used to periodically calibrate the free gyroscope. While this combination has certain benefits, it does not provide the unusually advantageous modes of operation and results as are afforded by the present invention. Among these are the enablement of very rapid surveying of boreholes; the lack of need for a free gyroscope to be periodically calibrated; and reduction in time required for surveying slanted boreholes, of particular advantage at depths where high temperatures are encountered.